1. Field of the Invention
The present invention relates to a boot voltage technology, and more particularly, to a voltage identification definition (VID) reference voltage generation circuit and a boot voltage generating method.
2. Description of Related Art
FIG. 1 is a block diagram of a conventional pulse width modulation voltage identification definition (PWMVID) reference voltage generator. Typically, a computer system 100 includes a PWMVID reference voltage generator 10. The PWMVID reference voltage generator 10 includes a resistor-capacitor network 20. The resistor-capacitor network 20 consists of resistors R1 to R4 and capacitor C1.
When the computer system 100 normally operates, a central processing unit (CPU) 30 generates a VID signal Svid with a duty cycle. The duty cycle D of this PWMVID signal Svid is equal to P2/P1, where P1 is a time duration of a cycle and P2 is a time duration of logic high level. After receiving the VID signal Svid, the PWMVID reference voltage generator 10 generates an output voltage Vout through the resistor-capacitor network 20. During normal operation, the output voltage Vout can be adjusted according to the VID signal Svid. The convenience of this technique includes that a slew rate of the VID signal Svid can be adjusted and the value of the output voltage Vout can also be adjusted. To a user, therefore, this technique has great freedom to adjust.
However, when the computer system 100 boots up, the central processing unit 30 has not transmitted the VID signal Svid to the PWMVID reference voltage generator 10 and, therefore, the output voltage Vout is a fixed value which cannot be adjusted. In other words, the output voltage Vout when the computer system 100 just boots up is determined by the reference voltage Vref and the resistors R3, R4.
Since the output voltage Vout when the computer system just boots up is fixed, the user is not able to decide the voltage value. The output voltage Vout when the computer system just boots up can also be referred to as a boot voltage. In some cases, the boot voltage needs to be a particular voltage value to execute a particular procedure. A particular procedure may not be executed immediately because the boot voltage is not equal to the particular voltage value.